Thinking about this again, these reference numbers could be for posts, not department, and they may not all be in Bomber Command HQ (Although I suspect most were). As an example, the (N.A.) documents which I am studying refer to a "W/Cdr F.C. Clarke, O.P.2". I have now been advised that W/Co. Clarke was a member of the Deputy Directorate of Organisation (Planning), which was part of the Department of the Air Member for Supply and Organisation (i.e. part of the Air Ministry?). So was HE O.P.2, or did he WORK IN O.P.2? The other references appear to be people (or teams) outside of the Air Ministry. As it is a Bomber Command file, I suspect that the were in Bomber Command HQ, but I'm really just guessing.........